CONAKRY, Feb 18 (Reuters) - At least one person was killed and six others injured in Guinea's capital Conakry on Tuesday when protests against frequent power cuts turned violent and the offices of a Brazilian construction firm were attacked, witnesses said.

The protests come as Guineans are growing increasingly frustrated with their government over the lack of development in their West African state, which despite its wealth in minerals remains mired in poverty after decades of misrule.

Legislative elections late last year completed a transition back to civilian rule after a 2008 military coup. President Alpha Conde is under intense pressure to deliver concrete change ahead of the next presidential vote in 2015.

The population lacks even the most basic social services and the country's infrastructure is in urgent need of improvement.

Security forces around the Lansanayah neighbourhood fired tear gas and beat protesters with batons, according to residents, some of whom said they heard gunfire.

"A youth died after being hit by a vehicle (belonging to the security forces). I saw the body," resident Aissatou Diallo told Reuters.

Some in the crowds attacked the local offices of Brazilian firm OAS, a privately-owned engineering and infrastructure development company which is involved in a number of public works projects. It was not clear why the company was targeted.

There was nobody available for comment at OAS in Conakry but a foreign diplomat, who declined to be identified, said one of the firm's local employees was injured.

Government spokesman Damantang Albert Camara confirmed the death of the protester but could not say if a vehicle belonging to the security forces had been involved.

The power cuts in a number of Conakry neighbourhoods in recent weeks have angered residents.

The government's plan to improve electricity production has been undermined by trouble connecting an extra 100 megawatts of capacity recently installed at a plant in Tombo to the Guinean power grid.

As a result, the government has had to hire an additional 50 MW of power supply from British firm Aggreko at a cost of $10 million every six months.

Guinea is the world's top bauxite exporter and is home to some of the world's largest untapped iron ore reserves.